1. Technical Field
The present invention relates generally to phase change memories.
2. Description of the Related Art
As is known, phase change memories use a class of materials that have the property of switching between two phases having distinct electrical characteristics, associated to two different crystallographic structures of the material, and precisely an amorphous, disorderly phase and a crystalline or polycrystalline, orderly phase. The two phases have resistivities of considerably different values.
Currently, the alloys of elements of group VI of the periodic table, such as Te or Se, referred to as chalcogenides or chalcogenic materials, can be used advantageously in phase change memory cells. The currently most promising chalcogenide is formed from an alloy of Ge, Sb and Te (Ge2Sb2Te5), which is now widely used for storing information on overwritable disks and has been also proposed for mass storage.
In the chalcogenides, the resistivity varies by two or more orders of magnitude when the material passes from the amorphous (more resistive) phase to the crystalline (more conductive) phase, and vice versa.
Phase change can be obtained by locally increasing the temperature. Below 150° C., both phases are stable. Starting from an amorphous state, and rising the temperature above 200° C., there is a rapid nucleation of the crystallites and, if the material is kept at the crystallization temperature for a sufficiently long time, it undergoes a phase change and becomes crystalline. To bring the chalcogenide back to the amorphous state it is necessary to raise the temperature above the melting temperature (approximately 600° C.) and then rapidly cool off the chalcogenide.
In a phase change memory, a plurality of memory cells are arranged in rows and columns to form an array. Each memory cell is coupled to a respective selection element, which may be implemented by any switching device, such as an ovonic threshold switch (OTS), a PN diode, a bipolar junction transistor or a MOS transistor. Each memory cell includes a memory region of a chalcogenide material in contact with a resistive electrode, also called heater. A storage element is formed at a contact area between the chalcogenide region and the heater. The heater is connected to a conduction terminal of the selection element.
In fact, from an electrical point of view, the crystallization temperature and the melting temperature are obtained by causing an electric current to flow through the resistive electrode in contact or close proximity with the chalcogenic material and thus heating the chalcogenic material by Joule effect. In particular, when the chalcogenic material is in the amorphous, high resistivity state (also called the reset state), applying a voltage/current pulse of a suitable length and amplitude and allowing the chalcogenic material to cool slowly causes the chalcogenic material changes its state and switch from a high resistivity to a low resistivity state (also called the set state). Vice versa, when the chalcogenic material is in the set state, applying a voltage/current pulse of suitable length and high amplitude causes the chalcogenic material to switch to the amorphous phase.